The vertebral or spinal column (spine, backbone) is a flexible assembly of vertebrae stacked on top of each other extending from the skull to the pelvic bone which acts to support the axial skeleton and to protect the spinal cord and nerves. The vertebrae are anatomically organized into four generalized body regions identified as cervical, thoracic, lumbar, and sacral; the cervical region including the top of the spine beginning in the skull, the thoracic region spanning the torso, the lumbar region spanning the lower back, and the sacral region including the base of the spine ending with connection to the pelvic bone. With the exception of the first two cervical vertebrae, cushion-like discs separate adjacent vertebrae, i.e. intervertebral discs.
The stability of the vertebral column during compression and movement is maintained by the intervertebral discs. Each disc includes a gel-like center surrounded by a fibrous ring. The gel-like center, i.e. nucleus pulposus, provides strength such that the disc can absorb and distribute external loads and contains a mixture of type II-collagen dispersed in a proteoglycan matrix. The fibrous ring, i.e. annulus fibrosus, provides stability during motion and contains laminated rings of type-I collagen. Thus, the annulus fibrosis and the nucleus pulposus are interdependent, as the annulus fibrosis contains the nucleus pulposus in place and the nucleus pulposus aligns the annulus fibrosus to accept/distribute external loads. The integrity of the composition and structure of the intervertebral disc is necessary to maintain normal functioning (of the intervertebral disc).
Many factors can adversely alter the composition and structure of the intervertebral disc, such as normal physiological aging, mechanical injury/trauma, and/or disease, resulting in impairment or loss of disc function. For example, the content of proteoglycan in the nucleus pulposus declines with age, thus, it follows that the ability of the nucleus pulposus to absorb water concurrently declines. Therefore, in normal aging the disc progressively dehydrates, resulting in a decrease in disc height and possible de-lamination of the annulus fibrosus. Mechanical injury can tear the annulus fibrosis allowing the gel-like material of the nucleus pulposus to extrude into the spinal canal and compress neural elements. Growth of a spinal tumor can impinge upon the vertebrae and/or disc potentially compressing nerves. Regardless of the cause, many of these disc pathologies become severe enough to require surgical intervention
In most cases, this surgical intervention is a partial or complete removal of the damaged intervertebral disc material (discectomy). While the discectomy eliminates the problems associated with the damaged disc, it creates a void in the intervertebral space that, if left empty, can de-stabilize and possibly collapse the vertebrae, negatively affecting the whole vertebral column.
The stabilization of the spine has been a focus of medical research for over 100 years. Early efforts at stabilization included spinal fusion. Spinal fusion was first used to treat the symptoms of spinal tuberculosis (Pott's disease) including the collapse of vertebral bodies and abnormal curvature of the spine. In this technique, bone tissue was harvested from the patient and applied to the dorsal spine. See Ryerson, Dr. Edwin W., The Journal of Bone and Joint Surgery; s2-12:259-264, 1914; wherein treatment of patients using the bone-graft procedure developed by Dr. Fred Albee is discussed. The entire contents are incorporated herein by reference. Although this basic technique is still in use today, bone grafts alone are slow to fuse and may not be anatomically correct even if fusion is achieved. Furthermore, such bone grafts are often not strong enough to maintain long-term stability of the spine.
In order to facilitate solid fusion and further stabilize vertebrae, various implantable hardware systems have been devised. These systems are applied to the surface of the spine and can compress the bone graft area to accelerate fusion. For example, an instrument, such as a rod, is placed generally parallel to a patient's back and attached to selected vertebrae by screws, clips, hooks, and/or clamps. Although this type of system initially offers strength, the hardware can loosen over time decreasing this strength.
In addition to spinal stabilization hardware, an extensive variety of devices for placement within the intervertebral space has been developed. For example, Harms, et al. (U.S. Pat. No. 4,820,305) provides a prosthesis in the form of a hollow cylindrical jacket which acts as a “place holder” between vertebrae. Buettner-Janz, et al. (U.S. Pat. No. 4,759,766) provides an intervertebral disc endoprosthesis including two endplates with an intermediate spacing device. Other devices combine structural elements with bone graft material. For example, Bagby (U.S. Pat. No. 4,501,269) discloses an implantable basket. In order to insert the basket, a hole is bored transversely across a joint. The bone shavings are collected and placed in a basket that is slightly larger than the hole. The basket is then driven into the hole to spread the bones. The entire contents of the '305, '766, and '269 patents are incorporated herein by reference.
Brantigan (U.S. Pat. No. 5,192,327) provides an annular implant with rigid surfaces having spaces for packing of bone graft material. The rigid surfaces engage the adjacent vertebral bodies to resist displacement. The annular implant may be inserted singly or multiple implants can be inserted as a set. The implants of the set can have different thicknesses and are internally grooved to receive a connecting bar to bind the implants together. The entire contents of the Brantigan '327 patent are incorporated herein by reference.
Despite the enormous progress in spinal therapy, chronic back pain remains a perplexing problem and the number of patients with back problems continues to rise.
Thus, ongoing efforts are desired to improve treatments and curtail the problem.